The present invention relates to a method and apparatus for testing CRT displays, such as computer displays, closed circuit television monitors, and broadcast televisions, and relates particularly to a method and apparatus for testing the effect of beam duty cycle variation on the accuracy of the display.
Modern CRT displays, such as those used in computer applications, often demand a high level of accuracy in the raster scan geometry, and if inaccuracies are present, it is important to be able to quantify such inaccuracies. Numerous systems are currently being utilized in which an analysis of data can be made by taking measurements directly off the image displayed on the screen, and the size and location of the image is critical to an interpretation of the data. For example, in computerized medical X-ray technology, tumors and organs are displayed directly on a computer CRT screen, so that accuracy in terms of size, location and shape are of paramount importance. Similarly, the use of CAD/CAM technology for design and production engineering requires that the projected image accurately represent the data which is being generated by the computer.
In generating an image on the screen of a CRT, the electron beam is caused to scan the screen in a conventional raster pattern comprising a plurality of interlaced or non-interlaced horizontal lines with the intensity of the beam varied to develop the image on the screen. With a background which is basically neutral or dark, the image on the screen will be developed by increasing the current of the electron beam so that the screen phosphors impinged by the high current beam will be excited thereby producing localized areas of high illumination. The illuminated areas form the image on the screen. By modulating the intensity of the beam as it scans the screen, various shades of gray can be produced, in a black and white display, and the overall character of the image display can have different levels of brightness. In many computer displays, the beam has only two intensities, a high intensity to form the image and a low intensity to form the background, although the polarity can be reversed so that the background is of high intensity, that is, highly illuminated, and the image of low intensity, that is, of a darker shade. Regardless of the character of the display, the more area of the screen that is highly illuminated, the greater will be the amount of current drawn from the high voltage supply by the beam. Similarly, if the display has an overall higher level of brightness, the electron beam will draw more current than will a low overall brightness display. In some computer displays, the "low intensity" areas are at a black level.
A problem which is inherent in almost all CRT displays, and which is more prevalent in lower quality displays, is a variation in the position, and possibly the size, of the image displayed on the screen, as a function of the amount of current drawn from the high voltage supply by the electron beam. If the high voltage supply of the CRT is limited in any way, as more current is drawn therefrom by the electron beam in generating a bright display image or a display image that has a relatively large amount of high illumination image area compared to the amount of background area, the voltage drops in the high voltage supply. When this voltage drops, it causes a shift in the magnetic field of the deflection control system for the electron beam thereby affecting centering of the image, and it may also cause a difference in the deflection angle, therefore affecting the size of the image. For example, consider an image generated on the screen, such as a technical drawing, wherein the lines of the drawing are highly illuminated and occupy a relatively low percentage of the screen and the background is at a low or black illumination level. If the polarity of the image is reversed, wherein the background is at a high illumination level and the lines of the drawing at a low illumination level, there will likely be a shift in the position of the image and also a possible change in its size. This is because when reversing the polarity, much more current will be drawn by the electron beam thereby causing a voltage drop in the high voltage supply. This change in voltage affects the strength of the beam control magnetic field thereby resulting in the displacement of the image on the screen.
This phenomenon can result in inaccuracies in the image displayed which may be quite significant, depending on the application. For example, if the CRT is used to display a computer generated image of a tumor in a computerized medical X-ray installation, and if the display is calibrated for a given level of illumination, the size and position of the tumor as display on the image will differ from its actual position if the level of illumination of the display is different from that at calibration.
If the display is utilized for representing a technical drawing in a CAD/CAM application, as the drawing is built up thereby adding more area of illumination, the duty cycle of the beam, that is, the average amount of current drawn from the high voltage supply, will increase thereby reducing the voltage in the high voltage supply and causing a variation in the size and location of the image. Similarly, if the polarity of a portion of the drawing is reversed, as is often done in computer displays to highlight the portion of the image displayed on the screen, this will increase the beam duty cycle and result in a variation in the size and location of the displayed image.
Although high quality displays will exhibit less of this effect than low quality displays, there is a need for the ability to quantify the amount of size and centering deviation for a particular display so that the deviation can be corrected for.